Ultraviolet curable coating compositions are well known in the art. Their use has been increasing rapidly in recent years because of coating industry efforts to reduce polluting effluents. Another major concern has been a reduction in the amount of energy required to ultimately cure the coatings to the desired degree of crosslinking. In non-pigmented coatings (clears) cure response has not been a problem in most instances apparently because clear coatings allow substantial through-and-through penetration of the film by the ultraviolet light. However, with pigmented coatings a different problem has become apparent, particularly with white, titanium dioxide-based coatings. In these instances, cure response properties have in many cases been so reduced that the coating did not cure below the surface leaving a non-cured liquid subsurface, or the coating remained soft and tacky. For this reason, there has been continuing development effort to provide new coating systems which minimize pollution problems and which are less energy-intensive.
U.S. Pat. No. 3,642,672 describes a polyester resin composition which cures under heat and pressure to form a rigid product which is characterized by an optically heterogeneous appearance. The composition comprises a thermoplastic cellulose ester, an unsaturated polyester, and a polymerizable monomer.
U.S. Pat. No. 3,907,656 describes a process in which the cure of a paint film having a binder that is polymerizable by irradiation is improved by providing entrapped opacifying gas bubbles within the paint film. The opacifying entrapped gas bubbles permit the use of less opacifying pigment such as titania. The film contains granules which entrap preformed opacifying gas bubbles. Optical opacity is achieved either by absorption of the incident light or by scattering of the incident light, or by a combination thereof.
U.S. Pat. No. 3,984,584 describes the production of opaque coatings and films with compositions which contain (1) photosensitized light curing condensation resins having double bond values of 0.18 to 0.45; ((2) fillers having low covering capacity; (3) film forming cellulose derivatives or vinyl chloride copolymers; and (4) a non-reactive solvent.
U.S. Pat. No. 3,993,798 describes a cured opaque coating with high hiding power produced from a normally transparent polyester paste filler composition. The composition consists essentially of unsaturated polyester, copolymerizable monomer, sensitizer, accelerator, non-reactive solvent and plasticizer, and filler material of low hiding power.
U.S. Pat. No. 4,005,244 describes an ultraviolet light curable coating composition comprising an ultraviolet curable component which shrinks upon curing, a non-curable component, and a photoinitiator. As the composition is cured by exposure to ultraviolet light, the curable component and the non-curable component phase separate and dentritic microfractures appear at phase boundries to provide opacity to the cured coatings.
There remains a need for new and improved ultraviolet light curable coating systems which satisfy more stringent pollution standards and energy conservation measures.
Accordingly, it is an object of this invention to provide a coating composition which is curable under minimal application of ultraviolet radiation.
It is another object of this invention to provide a low viscosity coating composition which is ultraviolet radiation curable as a 100 percent solids system.
It is a further object of this invention to provide a substantially transparent non-pigmented coating composition which is ultraviolet curable to a glossy white opaque enamel film on a substrate.
Other objects and advantages of the present invention shall become apparent from the accompanying description and examples.